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Thermodynamics of the Refrigeration Cycle

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Thermodynamics of the Refrigeration Cycle REFRIGERATION THERMODYNAMICS OF THE REFRIGERATION CYCLE BASIC KNOWLEDGE THERMODYNAMICS OF THE REFRIGERATION CYCLE Set-up and function of a compression The refrigeration cycle refrigeration system For operating media which can have different phases, Heat dissipation The refrigerant in a compression refrigeration system such as water or refrigerant, the T-s diagram looks Liquid during condensation flows through a closed cycle with the following four different. supercooled stations: Highg pressure It has an area on the left (grey), in which the operating Condensation Compres- Evaporation A medium is liquid and supercooled. In the centre (blue) sion Compression B there is a mixture of steam and liquid, the wet steam. Condensation C On the right of it (orange) the operating medium is in Expansion D s pure steam form and superheated. Expansionn ou The cooling takes place in the evaporator (A). The se Compressor The real refrigeration cycle with its typical phase tran- Evaporation Gaseous evaporation takes place at low pressures and tempera- Ga sitions can also be represented in this T-s diagram. power Wet steam superheated tures. Here the refrigerant absorbs heat from the envi- The cycle has many similarities to the familiar steam boiling temperature ronment and thus cools it. power cycle. The major difference is that the cycle is anticlockwise. Thus the processes of evaporation and The still cold refrigerant steam is aspirated by a d condensation and expansion and compression (pump- compressor (B) and subjected to higher pressure by ing) swap places. using mechanical energy. The refrigerant steam heats Liqui Refrigeration cycle in the T-s diagram up due to the compression. The enclosed area (green) corresponds to the compres- sor work added to the cycle. The hot refrigerant steam is cooled down in a condenser (C) and condenses while discharging heat to the envi- ronment. The liquid pressurised refrigerant is then expanded to Low pressure the low evaporation pressure in an expansion element The log p-h diagram for refrigerant (D) and returned to the evaporator. Heat absorption In the log p-h diagram the pressure p is plotted above the Liquid The refrigerant evaporates again and thus completes during evaporation supercooled enthalpy h. the circuit. In the centre (blue) is the wet steam area. Here the Cyclic process of a simple compression boilinggp temperature temperature corresponds to the boiling temperature for refrigeration system the pressure. The wet steam area is surrounded by limit curves with the steam content x=0.0 and x=1.0. Steam contenttt x To the left of it (grey) the refrigerant is liquid. The tempera- ture is below the boiling temperature for the pressure; the A cyclic process can be represented very clearly in the T-s diagram. Gaseous refrigerant is supercooled. superheated Here the temperature T of the operating medium is plotted above On the right (orange) the refrigerant is gaseous and the the entropy s. The area enclosed by the change of state of the temperature is above the boiling temperature. The refrig- Isothermal compression log p-h diagram for refrigerant operating medium corresponds to the work realised in the cyclic erant is superheated. process. Every refrigerant has its own log p-h diagram. The cyclic process with the highest possible efficiency is the Carnot cycle, here the enclosed area is a rectangle. This cycle is The log p-h diagram is better suited to represent the refrig- eration cycle than the T-s diagram and is therefore used Isothermal expansionp often used as a comparison cycle to describe the quality of the cyclic process. predominantly. The direction of the cyclic process in the T-s diagram determines Because energies exchanged with the refrigerant modify whether this is a heat pump cycle (refrigeration cycle) or a work the enthalpy h of the refrigerant, energy flows can be read machine cycle (steam power cycle). Refrigeration cycles are anti- directly from the diagram as horizontal lines. Ideal cyclic process (Carnot cycle) of a clockwise and the work represented by the green area is added gaseous medium in the T-s diagram to the cycle. 27 REFRIGERATION THERMODYNAMICS OF THE REFRIGERATION CYCLE BASIC KNOWLEDGE THERMODYNAMICS OF THE REFRIGERATION CYCLE The refrigeration cycle in the log p-h diagram The refrigerant The real refrigeration cycle consists of the following changes of state: Every cyclic process requires an operating medium which The different refrigerants are marked with an R followed in the refrigeration cycle is the refrigerant. In the refrigera- by a number. tion cycle the refrigerant has the purpose of transporting 1 – 2 polytropic compression on the The water often used in technical cycles is not suitable for heat. Here the high absorption of energy during evapora- condensation pressure (for comparison the refrigeration cycle. At the low temperatures prevail- tion or discharge of energy during the condensation of 1 – 2’ isentropic compression) ing in a refrigeration system the evaporation pressure is a liquid is utilised. To achieve this at the temperatures extremely low and there is a risk of the water freezing. prevailing in a refrigeration system at well manageable 2 – 2’’ isobaric cooling, deheating of the pressures, liquids with a low boiling point, such as differ- The use of CO is technically demanding. Due to its low superheated steam 2 ent fluorocarbons (FC), ammonia (NH3), carbon dioxide boiling temperature a very high pressure level results. (CO2) or hydrocarbons such as butane or propane, are This means that common components from refrigera- 2’’ – 3’ isobaric condensation used as operating medium. tion technology, such as valves, compressors or heat exchangers, cannot be used. 3’ – 3 isobaric cooling, supercooling of the For NH there are also special components, because liquid 3 Boiling materials containing copper are not resistant against Name temperature ammonia. 3 – 4 isenthalpic expansion to the evaporation pressure FC R134a Pure substance Ts = -26°C FC R404a Mixture Ts = -47℃ 4 – 1’ isobaric evaporation FC R407a Mixture Ts = -39...-45°C 1’ – 1 isobaric heating, superheating of the NH3 R717 Pure substance Ts = -33°C steam The refrigeration cycle in the log p-h diagram Isobutane R600a Pure substance Ts = -12℃ CO R744 Ts = -78°C In addition there are also pressure losses in the real refrigeration cycle, which means that evaporation and condensation 2 Pure substance are not exactly horizontal (isobaric). Energy considerations in the log p-h diagram Important for a good operation is the steam pressure The horizontal distances of the key cycle points in the curve of the operating medium. It should be gaseous log p-h diagram correspond to the enthalpy differences. In the at low pressures and at the desired cooling tempera- C simple refrigeration cycle without branched off mass flows these tures and liquid at high pressures and temperatures. result in the energy flows or capacities of the ideal system when The pressure levels should also be easy to manage multiplied with the refrigerant mass flow. The distances in the technically. log p-h diagram are therefore a direct measure for the energy flows exchanged. The diagram shows the steam pressure curve of the Temperature in ° Temperature well suited FC R134a. Typical freezing temperatures The distance 4 – 1 corresponds to the cooling capacity and is the of -26°C in the evaporator can be implemented with net capacity of the refrigeration system. The distance 1 – 2 is the pressures around 1bar while for condensing only a drive power exerted via the compressor. The distance 2 – 3 corre- pressure of 17bar at 60°C is required. sponds to the heat capacity discharged via the condenser. This is Pressure in bar a the waste heat of the refrigeration system. While in pure substances, such as NH3, propane and Steam pressure curve of FC R134a CO2,, the steam pressure curve is fixed, it can be From the ratio of the net capacity and the drive power the coef- adapted in FC within wide boundaries to meet require- ficient of performance COP can be calculated. ments by mixing different base grades. Energy flows in the refrigeration cycle h1 - h4 COP = cooling capacity absorbed h2 - h1 compressor drive power heat capacity discharged The coefficient of performance can be compared to the efficiency in a work machine. 29.
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